Method for manufacturing aluminium nitride single crystal

Abstract

 There is provided a method for manufacturing an aluminum nitride single crystal, the method including the steps of: preparing a raw material composition containing: aluminum oxide and/or an aluminum oxide precursor which is converted into aluminum oxide by heating, and aluminum nitride and/or an aluminum nitride precursor which is converted into aluminum nitride by heating; heating the raw material composition at 1600 to 2400°C to synthesize aluminum nitride; and causing crystal growth of the aluminum nitride to obtain an aluminum nitride single crystal. The method is capable of obtaining an aluminum nitride single crystal which is sufficiently large for practical use at low cost in a short time and has high productivity and wide usability.

Description

Background of the Invention and Related Art Statement

[0001] The present invention related to a method for manufacturing an aluminum nitride (AlN) single crystal useful as a dispersant (filler) for improving mechanical strength and heat radiation ability, or as a substrate material, heat radiating material, or structural material for parts for electronics or electrical machinery and appliances, particularly as a substrate material, heat radiating material, or structural material for parts for electronics or electrical machinery and appliances having high heating value such as a semi-conductive laser element and light-emitting diode.

[0002] In apparatuses and devices using parts having high heating value such as parts for electronics or electrical machinery and appliances, optical apparatuses, or OA apparatuses; quick radiation of heat generated is required. Therefore, as a substrate material and a heat radiation material (heat sink), which are brought into contact with these parts, or as a structural material (hereinbelow referred to as a "substrate material or the like") for these parts, there has conventionally been employed metallic material having high thermal conductivity and excellent heat radiation ability (e.g., aluminum (Al) and cupper (Cu)).

[0003] However, in recent years, apparatuses and device using such parts have tended to be minimized and densified and to raise output. A level of heat radiation ability required to a substrate material or the like has become higher. In addition, in some cases, properties such as mechanical strength and electric insulation, which cannot be imparted sufficiently to a metallic material, are required. In such a background, there is nowadays employed, as the substrate material or the like, aluminum nitride, which is excellent in various properties such as mechanical strength, thermal resistance, corrosion resistance, and electric insulation in addition to heat radiation ability.

[0004] It is general that a sintered body of aluminum nitride is used as a structural material for a substrate material or the like. However, attention has recently been paid to an aluminum nitride single crystal (bulk single crystal or whisker), which may be able to constitute a substrate material or the like having higher performance. A bulk single crystal is expected to be used as a substrate material or the like by the use of heat radiation ability thereof similarly to a sintered body, as a semi-conductive laser element or a light-emitting diode because of its wide energy-band gap (6.2eV), and as a substrate material for a semi-conductive laser element or a light-emitting diode because it has the same extent of lattice constant and thermal expansion coefficient as those of gallium nitride (GaN). Meanwhile, a whisker is also expected to be used as a dispersant (filler) for improving mechanical strength and heat radiation ability of metals and plastics because of its excellent mechanical strength and heat radiation ability.

[0005] There has been known, as methods for manufacturing an aluminum nitride single crystal, various methods such as nitriding, fluxing, chemical transportation, sublimation, and chemical vapor phase synthesis. However, since aluminum nitride is stable against heat, it hardly melts even under the condition of high temperature, and it is very difficult to grow a crystal largely. Therefore, there have been reported very few examples of manufacturing an aluminum nitride single crystal with a sufficient size for practical use as a substrate material or the like.

[0006] Some of the very few examples will be shown. For example, there has been reported a method for manufacturing a nitride single crystal, the method being characterized by mixing oxide powder, which reacts with the nitride under heating to decompose and gasify nitride, with nitride powder to obtain a mixed powder; heating the mixed powder at a temperature lower than sublimation temperature or melt temperature of the nitride in a nitrogen atmosphere or the like; subjecting the decomposed gasified component to crystal growth on a substrate from a vapor phase (see JP-A-10-53495). According to this method, it is considered that an aluminum nitride single crystal with dimensions of 10 mm × 10 mm or more and a thickness of 300 µm or more, which is sufficiently large as a bulk material, can be obtained.

[0007] There has also been reported a method for manufacturing an aluminum nitride bulk single crystal, characterized by bringing nitrogen into contact with a molten metal aluminum to form aluminum nitride in the molten body, and allowing the aluminum nitride to accumulate on a seed crystal which is in physical contact with the molten body (see 2003-505331). According to this method, it is considered that an aluminum nitride having a diameter of 1 inch (about 2.5 cm) or more can be obtained.

Summary of the Invention

[0008] However, it still leaves room for improvement from the viewpoint of productivity since crystal growth takes long time by the above methods though an aluminum nitride single crystal sufficiently large for practical use as a dispersant for improving mechanical strength and heat radiation ability or as a substrate material or the like of parts for electronics or electrical machinery and appliances.

[0009] To be concrete, it is considered that the method described in JP-A-10-53495 has low crystal-growth speed because of long maintaining time of about 24 hour at reaction temperature. Therefore, this method cannot give sufficiently satisfied crystal-growth speed in the case of considering practical use on an industrial level, and thereby it has a problem from the viewpoint of productivity and cost. Though the method described in 2003-505331 can achieve relatively high crystal-growth speed of about 1.6 mm/hour, the method requires an expensive special device provided with a complex mechanism such as a nitrogen gas injector, a crystal raiser, various controllers. Since this method cannot employ a widely used device and lacks wide usability, it is not necessarily suitable for practical use on an industrial level. That is, there has never been disclosed a method for manufacturing an aluminum nitride single crystal, the method being capable of obtaining an aluminum nitride single crystal which is sufficiently large for practical use at low cost in a short time and having high productivity and wide usability, and such a method is earnestly desired by the industrial world.

[0010] The present invention has been made to solve the aforementioned problems of conventional art and provides a method for manufacturing an aluminum nitride single crystal, the method being more effective in being capable of obtaining an aluminum nitride single crystal which is sufficiently large for practical use at low cost in a short time and having high productivity and wide usability in comparison with conventional methods.

[0011] The present inventors have eagerly studied to solve the aforementioned problems and, as a result, found out that the aforementioned problems can be solved by heating a raw material composition containing aluminum oxide and aluminum nitride at 1600 to 2400°C to synthesize aluminum nitride, which is then subjected to crystal growth. Thus, the present invention was led to completion. That is, according to the present invention, the following method for manufacturing an aluminum nitride single crystal is provided.

[0012] [1] A method for manufacturing an aluminum nitride single crystal, the method comprising the steps of:

preparing a raw material composition containing:

aluminum oxide (Al₂O₃) and/or an aluminum oxide precursor which is converted into aluminum oxide by heating, and

aluminum nitride (AlN) and/or an aluminum nitride precursor which is converted into aluminum nitride by heating;

heating the raw material composition at 1600 to 2400°C to synthesize aluminum nitride; and

causing crystal growth of the aluminum nitride to obtain an aluminum nitride single crystal.

[0013] [2] A method for manufacturing an aluminum nitride single crystal according to the above [1], wherein composition of the raw material composition satisfies the relation shown by the following formula (1):


where N_AO represents molar number of aluminum oxide, N_AO-pre represents molar number of aluminum oxide precursor in terms of aluminum oxide, N_AN represents molar number of aluminum nitride, and N_AN-pre represents molar number of aluminum nitride precursor in terms of aluminum nitride.

[0014] [3] A method for manufacturing an aluminum nitride single crystal, the method comprising the steps of:

preparing a raw material composition containing aluminum oxynitride and/or an aluminum oxynitride precursor which is converted into aluminum oxynitride by heating;

heating the raw material composition at 1600 to 2400°C to synthesize aluminum nitride; and

causing crystal growth of the aluminum nitride to obtain an aluminum nitride single crystal.

[0015] [4] A method for manufacturing an aluminum nitride single crystal according to the above [3], wherein at least one selected from the group consisting of Al₂₂O₃₀N₂, Al₂₃O₂₇N₅, Al₆O₃N₄, Al₇O₃N₅, and Al₉O₃N₇ is used as the aluminum oxynitride.

[0016] [5] A method for manufacturing an aluminum nitride single crystal according to any one of the above [1] - [4], wherein the synthesis and the crystal growth of the aluminum nitride is performed in the presence of carbon.

[0017] [6] A method for manufacturing an aluminum nitride single crystal according to any one of the above [1] - [5], wherein the synthesis and the crystal growth of aluminum nitride is performed in a graphite or nitride reaction container.

[0018] [7] A method for manufacturing an aluminum nitride single crystal according to any one of the above [1] - [6], wherein the synthesis and the crystal growth of the aluminum nitride is performed in a reaction atmosphere containing nitrogen gas and/or nitrogen compound gas with a partial pressure of the nitrogen gas and/or the nitrogen compound gas being 1kPa to 1MPa.

[0019] [8] A method for manufacturing an aluminum nitride single crystal according to the above [5], wherein the synthesis and the crystal growth of the aluminum nitride is performed under the condition that powdery carbon (carbon powder) is used as the carbon and that the carbon powder is mixed with the raw material composition.

[0020] [9] A method for manufacturing an aluminum nitride single crystal according to any one of the above [1] - [8], wherein a single crystal substrate is present in the reaction atmosphere to cause the crystal growth of the aluminum nitride on a surface of the single crystal substrate.

[0021] [10] A method for manufacturing an aluminum nitride single crystal according to the above [9], wherein the crystal growth of the aluminum nitride is performed under the condition that a difference in temperature between the raw material composition and the single crystal substrate is controlled to be 20°C or more.

[0022] The manufacture method of the present invention is more effective in being capable of obtaining an aluminum nitride single crystal which is sufficiently large for practical use at low cost in a short time and having high productivity and wide usability in comparison with conventional methods.

Brief Description of the Drawings

[0023] 

Fig. 1 is a graph showing a correlation between temperature and Gibbs free energy with respect to each of the reactions shown by the formulae (2) and (6) to (8).

Fig. 2 is a graph showing a correlation between temperature and Al₂O partial pressure with respect to each of the reactions shown by the formulae (2), (3) and (6) to (8).

Fig. 3 is a schematic sectional view of a heating furnace usable for a method for manufacturing an aluminum nitride single crystal of the present invention.

Fig. 4 is a microphotograph showing a platy aluminum nitride single crystal obtained by the manufacture method of Example 17.

Fig. 5 is a microphotograph showing a needle-shaped aluminum nitride single crystal obtained by the manufacture method of Example 3.

Detailed Description of the Invention

[0024] The best mode for carrying out a method for manufacturing an aluminum nitride single crystal of the present invention will hereinbelow be described specifically. However, a manufacture method of the present invention is by no means limited to the following embodiments.

[0025] In a method for manufacturing an aluminum nitride single crystal of the present invention, a raw material composition containing aluminum oxide and aluminum nitride is heated at 1600 to 2400°C to synthesize aluminum nitride, which is then subjected to crystal growth. Such a method is more effective in being capable of obtaining an aluminum nitride single crystal which is sufficiently large for practical use as a dispersant for improving mechanical strength or heat radiation ability, or as a substrate material, or the like, for parts for electronics or electrical machinery and appliances at low cost in a short time and having high productivity and wide usability in comparison with conventional methods. Incidentally, "aluminum nitride single crystal" in the present specification includes whisker besides aluminum nitride platy single crystal, needle-shaped single crystal, and bulk single crystal.

(1) Raw material composition:

[0026] In the manufacture method of the present invention, a raw material composition containing aluminum oxide is used. Though there are some cases of using metal aluminum instead of aluminum oxide, metal aluminum is not preferable in the following points. That is, though metal aluminum melts and volatilizes at low temperature, a nitride film is prone to be formed on a surface of a molten matter. Therefore, volatilization and synthesis reaction is hindered, which is a problem. In addition, though it is considered to be advantageous to conduct synthesis and crystal growth at high temperature to obtain a large single crystal, metal aluminum, which melts and volatilizes at low temperature is not suitable for such reaction.

[0027] On the other hand, in the case of using aluminum oxide as a raw material, aluminum nitride is formed by the reaction shown by the following formula (2). To be concrete, the reducing reaction shown by the following formula (2-1) proceeds to form Al₂O, which is a gaseous molecule, and then, as shown by the following formula (2-2), Al₂O (or a derivative thereof) reacts with nitrogen to form aluminum nitride. This method uses Al₂O as a source for an aluminum nitride single crystal. Since volatilization speed of Al₂O is relatively high, it is advantageous in the point of accelerating crystal-growth speed.

         Al₂O₃+3C+N₂ → 2AlN+3CO     (2)

         Al₂O₃+2C → 2Al₂O+2CO     (2-1)

         Al₂O+C+N₂ → 2AlN+CO     (2-2)

[0028] Since a manufacture method of the present invention includes a step of heating a raw material composition at high temperature, there may be used an aluminum oxide precursor which is converted into aluminum oxide by heating instead of a part of or the whole aluminum oxide contained in the raw material composition. Examples of an aluminum oxide precursor include aluminum hydroxide (Al(OH)₃), aluminum sulfate (Al₂(SO₄)₃), and aluminum alkoxide (Al(RO)₃, where R represents alkyl group). Among them, aluminum hydroxide can suitably be used from the viewpoint of requiring no apparatus for removing acid gas as a by-product unlike the case of aluminum sulfate and from the viewpoint of inexpensive cost for raw material unlike metal alkoxide.

[0029] A manufacture method of the present invention is characterized by using a raw material composition containing aluminum nitride besides the aforementioned aluminum oxide. Though an aluminum nitride formation process in the case of using a raw material composition containing aluminum oxide and aluminum nitride has not been clear yet, it is considered that Al₂O as a source of an aluminum nitride single crystal can be formed directly (see the following formula (3)) by reaction of aluminum oxide with aluminum nitride by allowing a raw material composition to contain aluminum nitride, or indirectly (see the following formulae (4) to (8)) via aluminum oxynitride formed by the reaction of aluminum oxide with aluminum nitride.

         Al₂O₃(s)+4AlN(s) → 3Al₂O(g)+2N₂(g)     (3)

         Al₂₂O₃₀N₂(s)+20C(s) → 10Al₂O(g)+2AlN(S)+20CO(g)     (4)

         Al₂₃O₂₇N₅(s)+18C(s) → 9Al₂O(g)+5AlN(s)+18CO(g)     (5)

         Al₆O₃N₄(s) → 3Al₂O(g)+2N₂(g)     (6)

         Al₇O₃N₅(s) → 3Al₂O(g)+AlN(s)+2N₂(g)     (7)

         AlgO₃N₇(s) → 3Al₂O(g)+3AlN(s)+2N₂(g)     (8)

[0030] Since a manufacture method of the present invention has a tendency of easily forming a crystalline nucleus of aluminum nitride in comparison with the case of using only aluminum oxide shown in the above formula (2), the method is suitable for manufacturing an aluminum nitride single crystal. According to such a method, crystal growth of aluminum nitride is accelerated, and an aluminum nitride single crystal which is sufficiently large for practical use can be manufactured in a short time. Of all, a method in which Al₂O is indirectly formed via aluminum oxynitride formed by the reaction of aluminum oxide with aluminum nitride as shown in the above formulae (4) to (8) is preferable from the viewpoint of stably manufacturing highly qualified aluminum nitride single crystal.

[0031] Further, a manufacture method of the present invention can give a colorless, transparent aluminum nitride single crystal and has advantage in being hard to lower thermal conductivity of the single crystal. This is because of a low volume ratio of carbon monoxide formed as a by-product when aluminum nitride is formed as shown in the above formulae (3) to (8). In particular, a method where Al₂O is indirectly formed via aluminum oxynitride as shown in the above formulae (6) to (8) is preferable because it has a very low volume ratio of carbon monoxide formed as a by-product. On the other hand, in the case of using only aluminum oxide as a raw material, a large amount of carbon monoxide is formed in the reaction system and causes solid solution in a formed aluminum nitride single crystal, and thereby a single crystal is sometimes blue-colored or deteriorated in thermal conductivity.

[0032] Incidentally, there is present an example of using an aluminum nitride coarse crystal alone as an aluminum source upon aluminum nitride synthesis. However, such a method differs in technical concept from a method of the present invention and has the following problems. That is, since an aluminum nitride coarse crystal has high sublimation temperature and low speed of sublimation, it is difficult to obtain sufficient crystal-growth speed at low temperature. Since crystal growth at high temperature requires a special device capable of coping with such high temperature, the method has a problem of being defective in wide usability. In addition, in the case of using aluminum nitride alone as a raw material, there is a problem of high production cost due to high cost of the raw material.

[0033] On the other hand, it has conventionally been a technical common sense that a method using a mixture of aluminum oxide and aluminum nitride as a raw material is not a preferable method because an aluminum oxynitride as a by-product is expected judging from constitutional diagrams of these materials.

[0034] However, the present inventors found a method where an aluminum nitride single crystal can be obtained without remaining aluminum oxynitride even when a mixture of aluminum oxide and aluminum nitride is used as a raw material. Such a method can accelerate crystal-growth speed and lower crystal-growth temperature and has the advantage in being capable of using a widely used device. Further, it has the advantage of low production cost due to low raw material cost over a method using an aluminum nitride coarse crystal alone.

[0035] In addition, since a method of the present invention contains a step of heating a raw material composition at high temperature, there may be employed an aluminum nitride precursor which can be converted into aluminum nitride by heating instead of a part of or the whole aluminum nitride contained in the raw material. Examples of an aluminum nitride precursor include metal aluminum, aluminum carbide (e.g., Al₄C₃ or Al₂C₆), Boehmite (AlO(OH) or Al₂O₃·H₂O), and aluminum chloride (AlCl₃). Aluminum nitride can preferably be used from the viewpoint of stability and handleability.

[0036] Further, in a method of the present invention, a raw material composition may contain a material where aluminum oxide and aluminum nitride coexist like partially oxidized aluminum nitride (for example, aluminum nitride where a surface portion of an aluminum nitride powder is oxidized) or like partially nitrided aluminum oxide. Using such a raw material composition, synthesis and crystal growth of aluminum nitride is possible under similar conditions.

[0037] Though forms of aluminum oxide and aluminum nitride are not particularly limited, it is general to use powder from the viewpoint of being easily mixed and easy progress in reaction.

[0038] Though composition of a raw material composition is not particularly limited, it preferably satisfies the following formula (1), more preferably satisfies the following formula (9), and particularly preferably satisfies the following formula (10).






(where N_AO represents molar number of aluminum oxide, N_AO-pre represents molar number of aluminum oxide precursor in terms of aluminum oxide, N_AN represents molar number of aluminum nitride, and N_AN-pre represents molar number of aluminum nitride precursor in terms of aluminum nitride).

[0039] When ratio of aluminum oxide exceeds the above range, partial pressure of carbon monoxide rises during crystal formation, which is prone to cause coloring or deterioration in thermal conductivity of a formed aluminum nitride single crystal, which is not preferable. On the other hand, when ratio of aluminum nitride exceeds the above range, it raises raw material-volatilizing temperature and crystal-forming temperature in addition to increase in amount of aluminum nitride which does not contribute to the reaction. Therefore, it is not preferable from the viewpoint of being prone to lower yield and crystal-growth speed.

[0040] The above raw material composition can be obtained by weighing each of raw materials and mixing them according to a conventionally known mixing method. As a mixing method, there is a mixing method using a conventionally known mixer or grinder (e.g., ball mill and media-type grinder). In the case of small amount of raw material composition, mixing may be conducted by a method where the raw materials are put in a bag, container, or the like, to be shaken (hand mixing).

[0041] In a method of the present invention, it is also preferable to use a raw material composition containing aluminum oxynitride instead of a raw material composition containing aluminum oxide and aluminum nitride. With respect to a raw material composition containing aluminum oxynitride, synthesis and crystal growth of aluminum nitride can be conducted under similar conditions to those for a raw material composition containing aluminum oxide and aluminum nitride.

[0042] In the present specification, "aluminum oxynitride" means a compound shown by the general formula, Al_xO_yN_z (where x, y, and z each denotes an integer of 1 or higher), for example, Al₂₂O₃₀N₂, Al₂₃O₂₇N₅, Al₆O₃N₄, Al₇O₃N₅, and Al₉O₃N₇.
Compounds where aluminum oxide or aluminum nitride is solid-solved in these compounds are also included in "aluminum oxynitride" of the present specification.

[0043] In the case of using a raw material composition containing aluminum oxide and aluminum nitride, an aluminum nitride crystal is formed by a gas component volatilized from a liquid phase. Meanwhile, in the case of using a raw material composition containing aluminum oxynitride, an aluminum nitride crystal is formed by a gas component volatilized from a solid phase, which enables to manufacture a highly purified aluminum nitride single crystal. In particular, in the case of using aluminum oxynitride powder as a raw material, it has the advantage of rapid crystal growth because of its large surface area.

[0044] It is preferable to use a raw material composition containing aluminum oxynitride because formation of carbon monoxide as a by product is suppressed and coloring or deterioration in thermal conductivity of a formed aluminum nitride single crystal is effectively inhibited. This is because of low volume ratio of carbon monoxide formed as a by-product when aluminum nitride is formed as shown in the above formulae (3) to (8). Particularly, it is preferable to use aluminum oxynitride shown in the above formulae (6) to (8) (Al₆O₃N₄, Al₇O₃N₅, or Al₉O₃N₇) because volume ratio of carbon monoxide formed as a by-product becomes very low.

[0045] Further, it is preferable to use a raw material composition containing aluminum oxynitride also from the viewpoint of being capable of progress in reaction at lower temperature. Fig. 1 is a graph showing a correlation between temperature and Gibbs free energy (□G) with respect to each of the reactions shown by the formulae (2) and (6) to (8). As clear from this graph, the reaction shown by the above formulae (6) to (8) using aluminum oxynitride as a raw material makes progress from around 1100°C. On the other hand, the reaction shown by the above formula (2) using aluminum oxide as a raw material does not make progress till the temperature reaches around 1400°C.

[0046] Furthermore, it is preferable to use aluminum oxynitride shown in the above formulae (6) to (8) (Al₆O₃N₄, Al₇O₃N₅, or Al₉O₃N₇) because deviation in raw material composition due to nitriding or the like of the raw material composition is hardly caused and the raw material composition is hardly influenced by reaction temperature during crystal formation, and thereby a highly qualified aluminum nitride single crystal can stably be manufactured. Fig. 2 is a graph showing a correlation between temperature and Al₂O partial pressure with respect to each of the reactions shown by the formulae (2), (3) and (6) to (8). As clear from this graph, use of aluminum oxynitride shown by the above formulae (6) to (8) as the raw material makes inclination of the straight line gentle and can stably supply Al₂O as a source of an aluminum nitride single crystal even if temperature changes. That is, the material is hardly influenced by reaction temperature, and a highly qualified aluminum nitride single crystal can stably be manufactured. On the other hand, in the reaction of the above formula (2) using aluminum oxide as the raw material and the reaction of the above formula (3) using aluminum oxide and aluminum nitride as the raw material, a supply amount of Al₂O is changed in accordance with temperature change. Therefore, quality of an aluminum nitride single crystal is possibly influenced by reaction temperature.

(2) Synthesis and crystal growth of aluminum nitride

[0047] In a method of the present invention, the aforementioned raw material composition is heated at 1600 to 2400°C. By such conditions, aluminum nitride to be aimed at is synthesized and the aluminum nitride is subjected to crystal growth to give an aluminum nitride single crystal.

[0048] Since reaction is conducted in a non-oxygen atmosphere to inhibit formed aluminum nitride from being oxidized in a method of the present invention, it is general to cause reaction and crystal growth in an inert gas-containing atmosphere such as nitrogen, helium, and argon. In a method of the present invention, a nitrogen compound capable of serving as a nitrogen source of aluminum nitride or aluminum oxynitride is contained in the raw material composition. It is not necessary to conduct synthesis and crystal growth in an atmosphere containing nitrogen gas or nitrogen compound gas (e.g., anmonia (NH₃)), which can be a nitrogen source. However, it is preferable to conduct synthesis and crystal growth in an atmosphere containing nitrogen gas or nitrogen compound gas from the view point of stabilizing formed aluminum nitride and accelerating formation reaction of aluminum nitride. Among them, it is particularly preferable to conduct synthesis and crystal growth in an atmosphere containing nitrogen gas in that it does not require an apparatus for removing a nitrogen compound such as ammonia discharged from the reaction system.

[0049] Though pressure of the reaction atmosphere is not particularly limited, the reaction is conducted in a reaction atmosphere under a partial pressure of nitrogen gas and/or nitrogen compound gas of preferably 1kPa to 1MPa or less, more preferably 10 to 500kPa, and particularly preferably 10 to 200kPa. When the partial pressure is below the aforementioned range, formed aluminum nitride may become unstable and decompose, thereby crystal-growth speed of aluminum nitride may be lowered, which is not preferable. When the partial pressure is above the aforementioned range, raw materials such as aluminum oxide and aluminum oxynitride are easily nitrided, and it is apprehended that the effect of accelerating crystal growth is not exhibited sufficiently, which is not preferable.

[0050] In addition, though oxygen concentration of the reaction atmosphere is not particularly limited, it is considered that it is preferable to control the concentration generally 1 mol% or less. When it exceeds the range, there is a possibility of deterioration in quality of the aluminum nitride crystal due to solid solution of oxygen in the crystal obtained, which is not preferable.

[0051] In a method of the present invention, it is preferable to conduct reaction of synthesis and crystal growth of aluminum nitride in the presence of carbon. This is because carbon as a aluminum oxide reducing agent is made present in the reaction system because aluminum oxide is used as an aluminum source of aluminum nitride.

[0052] A method of conducting reaction in the presence of carbon is exemplified by a method using a graphite reaction container or heating apparatus. To be concrete, there may suitably be employed a method using a graphite crucible as a reaction container or a method using a graphite heater as a heating apparatus. It is one of preferably embodiment that synthesis and crystal growth of aluminum nitride is conducted in the condition that carbon powder as carbon is mixed with the raw material composition.

[0053] It is also preferable that synthesis and crystal growth of aluminum nitride is conducted in a nitride reaction container. This is because nitride generally has high melting point and can be used stably even for a reaction under high-temperature conditions as in the present invention. In the case of using a graphite crucible, there sometimes causes blue-coloring of the single crystal due to solid solution of carbon dioxide when molar ratio of aluminum oxide is high in the raw material composition (e.g., aluminum oxide : aluminum nitride = 90 : 10). However, in the case of a nitride crucible, such a defect can effectively be suppressed. Examples of a nitride include boron nitride (BN), titanium nitride (TiN), zirconium nitride (ZrN), and aluminum nitride (AlN). Among them, boron nitride, which is more stable, can preferably be used.

[0054] Temperature of reaction in a method of the present invention is necessarily in the range from 1600 to 2400°C, preferably in the range from 2000 to 2300°C, more preferably in the rage from 2100 to 2300°C. When temperature is below the aforementioned range, it is apprehended that growth speed of an aluminum nitride crystal is lowered because movement of atoms or molecules concerned in synthesis of aluminum nitride becomes slow, which is not preferable. On the other hand, when temperature exceeds the range, the temperature approaches decomposition temperature of aluminum nitride, and thereby formed aluminum nitride may become unstable and may be decomposed, which is not preferable.

[0055] The optimum temperature conditions in a method of the present invention varies in the range from 1600 to 2400°C depending on production conditions such as kind of the raw material composition. Under the production conditions, it is preferable to set the temperature within the range from the temperature where Al₂O gas sufficiently volatilizes to the temperature where formed aluminum nitride becomes unstable and decomposes. At this time, it is one of preferable embodiments that another substance is added to lower the temperature where Al₂O gas sufficiently volatilizes. Examples of another substance to lower the temperature where Al₂O gas sufficiently volatilizes include alkali metal compounds, alkaline earth metal compounds, and transition metal compounds.

[0056] In addition, in the case of using a raw material composition containing aluminum oxide (or aluminum oxide precursor) and aluminum nitride (or aluminum nitride precursor), it is preferable that temperature-rising speed is lowered in order to accelerate formation of aluminum oxynitride in the system. To be concrete, temperature-rising speed within the range from 1000 to 2000°C is preferably 0.5 to 50°C/min., and more preferably 2 to 20°C/min.

[0057] In addition, in a method of the present invention, it is preferable to make a single crystal substrate present in a reaction atmosphere for crystal growth of aluminum nitride on a surface of the single crystal substrate. This crystal growth is epitaxial growth having the same orientation as a single crystal constituting the single crystal substrate and contributes to manufacture of a highly qualified aluminum nitride single crystal having a large size sufficient for use as substrate material, or the like, for parts for electronics or electrical machinery and appliances and having few crystal defects. There may suitably be used, as a single crystal constituting the single crystal substrate, sapphire or silicon carbide (SiC), not to mention aluminum nitride.

[0058] In the case of thus employing a method where crystal growth of aluminum nitride is caused on a surface of a single crystal substrate, it is more preferable to control temperature difference between the raw material composition and the single crystal substrate to be 20°C or more.

[0059] By controlling the temperature difference to be 20°C or more, there can be obtained the effects in accelerating crystal growth of aluminum nitride and in suppressing damage of the single crystal substrate by heat. On the other hand, when the temperature difference is below the aforementioned range, a part of highly reactive atoms or molecules which are concerned in synthesis of aluminum nitride react with the single crystal substrate, and the single crystal substrate may be damaged, which is not preferable. From the viewpoint of suppressing damage in the single crystal substrate, the temperature difference does not have particular upper limit. However, the temperature difference is preferably controlled at 300°C or less, more preferably at 200°C or less, in that a cooling apparatus or the like is not required separately and that the method can be carried out with a widely used device.

[0060] Incidentally, there is no particular limitation to a method for controlling the temperature difference between the raw material composition and the single crystal substrate to be 20°C or more. An example of the method is a method where two or more heaters each having independent control system are used to control temperature around the reaction container high and temperature around the single crystal substrate low, thereby giving a temperature inclination in the reaction atmosphere.

[0061] A method of the present invention is, as described above, more effective in being capable of obtaining an aluminum nitride single crystal which is sufficiently large for practical use at low cost in a short time and having high productivity and wide usability in comparison with conventional methods. It is possible to obtain a platy single crystal having the maximum outer diameter of over 1 cm in a short time of about two hours by setting appropriate conditions.

Example

[0062] A method for manufacturing an aluminum nitride single crystal of the present invention is hereinbelow described specifically with Examples. However, the present invention is by no means limited by these Examples. In the manufacture methods in Examples and Comparative Examples, each of the obtained aluminum nitride single crystal was evaluated for size. As a size of each of the single crystals, length and the maximum outer diameter in the case of a needle-shaped single crystal or the maximum outer diameter and thickness in the case of a platy single crystal were evaluated according to the following method. In the present specification and tables, "needle-shaped single crystals" include trigonal prism-shaped single crystals besides needle-shaped single crystals.

[Needle-shaped single crystal]

[0063] With respect to a needle-shaped single crystal, a product after the completion of reaction was measured for length by visual observation and for the maximum outer diameter by observation with a scanning electron microscope (SEM). In the present specification, "the maximum outer diameter" means length of the longest side in the case that a section perpendicular to the longitudinal direction is triangular, length of the longest diagonal line in the case that a section perpendicular to the longitudinal direction is polygonal (rectangle or more), length of a diameter in the case that a section perpendicular to the longitudinal direction is circular, length of the major axis in the case that a section perpendicular to the longitudinal direction is oval. When the observable maximum crystal is a needle-shaped crystal having a length of 20 mm or more and the maximum outer diameter of 0.5 mm or more, evaluation was given as "excellent". When the observable maximum crystal is a needle-shaped crystal having a length of 10 mm or more and the maximum outer diameter of 0.5 mm or more, evaluation was given as "good". When the observable maximum crystal is a needle-shaped crystal having a length of 5 mm or more and the maximum outer diameter of 0.2 mm or more, evaluation was given as "fair". When a needle-shaped crystal having a length of 5 mm or more and the maximum outer diameter of 0.2 mm or more is not observed, evaluation was given as "bad".

[Platy single crystal]

[0064] With respect to a platy single crystal, a product after the completion of reaction was measured for the maximum outer diameter by visual observation and for thickness by observation with an optical microscope. When the observable maximum crystal is a platy single crystal having the maximum outer diameter of 10 mm or more and a thickness of 0.5 mm or more, evaluation was given as "excellent". When the observable maximum crystal is a platy single crystal having the maximum outer diameter of 5 mm or more and a thickness of 0.2 mm or more, evaluation was given as "good". When the observable maximum crystal is a platy single crystal having the maximum outer diameter of 1 mm or more and a thickness of 0.1 mm or more, evaluation was given as "fair". When a platy single crystal having the maximum outer diameter of 1 mm or more and a thickness of 0.1 mm or more is not observed, evaluation was given as "bad".

(Examples 1 to 19, Comparative Examples 1 to 6)

[0065] Each of raw material compositions was prepared by mixing aluminum oxide with aluminum nitride at a molar ratio shown in Table 1. For the mixing, aluminum oxide and aluminum nitride were put in a polyethylene bag, which was then shaken.

[0066] With respect to raw material compositions prepared as described above, synthesis and crystal growth of aluminum nitride was conducted with a heating furnace 20 provided with carbon heaters 22 and a vacuum chamber 30 as shown in Fig. 3. First, a raw material composition 10 was filled into the first crucible 12 (made of boron nitride and having an inner diameter of 40 mm and a height of 50 mm) having an opening in the upper portion thereof. The first crucible 12 was put in the second crucible 14 (made of carbon and having an inner diameter of 90 mm and a height of 70 mm) which also has an opening in the upper portion thereof and which is larger than the first crucible. The second crucible 14 was sealed at the opening with a lid 16 (made of carbon and having an outer diameter of 100 mm and a thickness of 5 mm) and then disposed on a stage 24 inside the vacuum chamber 30 of a heating furnace 20. Incidentally, carbon powder was not added to a raw material composition as a carbon source in any of Examples and Comparative Examples, and only the carbon heaters 22, the crucible 14, and the lid 16 were served as a carbon source. A single crystal substrate was not present in a reaction atmosphere, and a crystal was made to grow on an inner wall of the crucible 12.

[0067] Gas was discharged with a vacuum pump from a gas exhaust port 28 to reduce an internal pressure of the vacuum chamber 30 to 5×10^-4Pa. Then, nitrogen gas or argon gas was supplied in the chamber from a gas induction port to adjust to give an atmospheric pressure shown in Table 1. Then, the temperature was raised up to a temperature shown in Table 1 at a temperature-rising speed of 20°C/min. After the temperature was maintained for two hours, the furnace was cooled down to try synthesis and crystal growth of aluminum nitride. The result was shown in Table 1.

[Evaluation]

[0068] As shown in Table 1, according to a manufacture method of Examples 1 to 19, there could be obtained a needle-shaped crystal (including a trigonal prism-shaped crystal) having a length of 5 mm or more and the maximum outer diameter of 0.2 mm or more and a platy crystal having the maximum outer diameter of 1 mm or more and a thickness of 0.1 mm or more, which were sufficiently large for practical use as fillers. These crystals were analyzed by energy dispersive X-ray analysis (EDX) to found that there was no oxygen in the crystals and that elements constituting the crystals were only aluminum and nitrogen. Further, it was confirmed by X-ray diffraction (XRD) that the crystals were aluminum nitride crystals.

[0069] Further, by observation with SEM, it was presumed that the needle-shaped crystals were single crystals grown in the (0001) direction because of their hexagonal shape. In addition, it was confirmed by X-ray diffraction that the platy crystals were single crystals whose plate surfaces face (0001) direction. Though growth surfaces of trigonal prism-shaped single crystals are complex and has not yet made clear, one of crystal surfaces faces (102) direction, and a X-ray rocking curve full width at half maximum for the surface was 18 second, which is an excellent value. That is, it was confirmed that the crystals were excellent in crystallinity and had very few defects. Further, blue-coloring due to solid-solution of carbon monoxide was not found in these single crystals.

[0070] Among the methods of Examples 1 to 19, the methods of Examples 5, 6, 11, and 15 to 17 could give platy single crystals each having the maximum outer diameter of 10 mm or more and a thickness of 0.5 mm or more, which is sufficiently large for practical use as a substrate material or the like of parts for electronics or electrical machinery and appliances. Fig. 4 is a microphotograph showing a platy aluminum nitride single crystal obtained by the manufacture method of Example 17. In addition, also the methods of Examples 2, 4, 7 to 10, 12 to 14, 18, and 19 could give platy single crystals each having the maximum outer diameter of 5 mm or more and a thickness of 0.2 mm or more, which is sufficiently large for practical use as a substrate material, showing relatively good results. By investigating manufacture conditions such as using a single crystal substrate in a reaction atmosphere in detail in the future, it can be expected to obtain larger platy single crystals.

[0071] On the other hand, the methods of Examples 2, 3, 5, 10, and 15 could give needle-shaped crystals each having a length of 20 m or more and the maximum outer diameter of 0.5 mm or more, showing excellent results. Fig. 5 is a microphotograph showing a needle-shaped aluminum nitride single crystal obtained by the manufacture method of Example 3. Also, the methods of Examples 1 and 17 could give needle-shaped crystals each having a length of 10 mm or more and the maximum outer diameter of 0.5 mm or more, showing relatively good results.

[0072] Incidentally, there were advantageously formed platy single crystals by the methods of Examples 6, 11, 15, and 17, and needle-shaped crystals by the methods of Examples 2, 3, and 10. It was found that it is possible to control the shape of a single crystal by manufacture conditions.

[0073] On the other hand, in Comparative Examples 1 to 6 using aluminum oxide or aluminum nitride alone as a raw material composition, there was not confirmed formation of any needle-shaped single crystal having a length of 5 mm or more and the maximum outer diameter of 0.2 mm or more or any platy single crystal having the maximum outer diameter of 1 mm or more and a thickness of 0.1 mm or more at all.

(Examples 20 to 34)

[0074] Synthesis and crystal growth of aluminum nitride was tried in the same manner as in Example 1 except that previously synthesized aluminum oxynitride (synthetic ALON) was used as a raw material. Conditions for synthesis and crystal growth of aluminum nitride were as described in Table 2. The results are shown in Table 2.

[Evaluation ]

[0075] As shown in Table 2, according to the methods of Examples 20 to 34, regardless of conditions, needle-shaped crystals (including trigonal prism-shaped crystals) having a length of 10 mm or more and the maximum outer diameter of 0.5 mm ore more and platy crystals having the maximum outer diameter of 1 mm or more and a thickness of 0.1 mm or more, which are sufficiently large for practical use as a filler, in a shot time of 2 hours. These crystals were subjected to energy dispersive X-ray analysis (EDX), and it was found that there was no oxygen in the crystals and that elements constituting the crystals were only aluminum and nitrogen. Further, it was confirmed by X-ray diffraction (XRD) that the crystals were aluminum nitride crystals.

[0076] Further, by observation with SEM, it was presumed that the needle-shaped crystals were single crystals grown in the (0001) direction because of their hexagonal shape. In addition, it was confirmed by X-ray diffraction that the platy crystals were single crystals whose plate surfaces face (0001) direction. Though growth surfaces of trigonal prism-shaped single crystals are complex and has not yet made clear, one of crystal surfaces faces (102) direction, and a X-ray rocking curve full width at half maximum for the surface was 18 second, which is an excellent value. That is, it was confirmed that the crystals were excellent in crystallinity and had very few defects. Further, blue-coloring due to solid-solution of carbon monoxide was not found in these single crystals.

[0077] Among the methods of Examples 20 to 34, the methods of Examples 20, 23 to 27, and 29 to 34 could give platy single crystals each having the maximum outer diameter of 10 mm or more and a thickness of 0.5 mm or more, which is sufficiently large for practical use as a substrate material or the like of parts for electronics or electrical machinery and appliances, showing excellent results. In addition, also the methods of Examples 21, 22, 28, and 29 could give needle-shaped single crystals each having a length of 20 mm or more and the maximum outer diameter of 0.5 mm or more, which is sufficiently large for practical use as a substrate material, showing excellent results.

[0078] A method for manufacturing an aluminum nitride single crystal of the present invention can preferably be used for manufacturing an aluminum nitride single crystal (bulk single crystal or whisker) hopeful as a dispersant (filler) for improving mechanical strength and heat radiation ability of metals or plastics, or as a substrate material, heat radiating material, or structural material for parts for electronics or electrical machinery and appliances, as well as a semi-conductive laser element and light-emitting diode.

Claims

1. A method for manufacturing an aluminum nitride single crystal, the method comprising the steps of:

preparing a raw material composition containing:

aluminum oxide (Al₂O₃) and/or an aluminum oxide precursor which is converted into aluminum oxide by heating, and

aluminum nitride (AlN) and/or an aluminum nitride precursor which is converted into aluminum nitride by heating;

heating the raw material composition at 1600 to 2400°C to synthesize aluminum nitride; and

causing crystal growth of the aluminum nitride to obtain an aluminum nitride single crystal.

2. A method for manufacturing an aluminum nitride single crystal according to Claim 1, wherein composition of the raw material composition satisfies the relation shown by the following formula (1):


where N_AO represents molar number of aluminum oxide, N_AO-pre represents molar number of aluminum oxide precursor in terms of aluminum oxide, N_AN represents molar number of aluminum nitride, and N_AN-pre represents molar number of aluminum nitride precursor in terms of aluminum nitride.

3. A method for manufacturing an aluminum nitride single crystal, the method comprising the steps of:

preparing a raw material composition containing aluminum oxynitride and/or an aluminum oxynitride precursor which is converted into aluminum oxynitride by heating;

heating the raw material composition at 1600 to 2400°C to synthesize aluminum nitride; and

causing crystal growth of the aluminum nitride to obtain an aluminum nitride single crystal.

4. A method for manufacturing an aluminum nitride single crystal according to Claim 3, wherein at least one selected from the group consisting of Al₂₂O₃₀N₂, Al₂₃O₂₇N₅, Al₆O₃N₄, Al₇O₃N₅, and Al₉O₃N₇ is used as the aluminum oxynitride.

5. A method for manufacturing an aluminum nitride single crystal according to any one of Claims 1 - 4, wherein the synthesis and the crystal growth of the aluminum nitride is performed in the presence of carbon.

6. A method for manufacturing an aluminum nitride single crystal according to any one of Claims 1 - 5, wherein the synthesis and the crystal growth of aluminum nitride is performed in a graphite or nitride reaction container.

7. A method for manufacturing an aluminum nitride single crystal according to any one of Claims 1 - 6, wherein the synthesis and the crystal growth of the aluminum nitride is performed in a reaction atmosphere containing nitrogen gas and/or nitrogen compound gas with a partial pressure of the nitrogen gas and/or the nitrogen compound gas being 1kPa to 1MPa.

8. A method for manufacturing an aluminum nitride single crystal according to Claim 5, wherein the synthesis and the crystal growth of the aluminum nitride is performed under the condition that powdery carbon (carbon powder) is used as the carbon and that the carbon powder is mixed with the raw material composition.

9. A method for manufacturing an aluminum nitride single crystal according to any one of Claims 1 - 8, wherein a single crystal substrate is present in the reaction atmosphere to cause the crystal growth of the aluminum nitride on a surface of the single crystal substrate.

10. A method for manufacturing an aluminum nitride single crystal according to Claim 9, wherein the crystal growth of the aluminum nitride is performed under the condition that a difference in temperature between the raw material composition and the single crystal substrate is controlled to be 20°C or more.

Drawing